In recent years, there has been a tremendous expansion of the capabilities for the genetic engineering of plants. Many transgenic dicotyledonous plant species have been obtained. However, many species of plants, especially those belonging to the Monocotyledonae and particularly the Gramineae, including economically important species such as corn, wheat and rice, have proved to be very recalcitrant to stable genetic transformation.
Difficulties have been encountered in achieving both: a) integrative transformation of monocot plant cells with DNA (i.e., the stable insertion of DNA into the nuclear genome of the monocot plant cells) and b) regeneration from transformed cells of phenotypically normal monocot plants, such as phenotypically normal, fertile adult monocot plants. It has been suggested that such difficulties have been predominantly due to the nonavailability of monocot cells that are competent with respect to: 1) DNA uptake, 2) integrative transformation with the taken-up DNA, and 3) regeneration of phenotypically normal, monocot plants from the transformed cells (Potrykus (1990) Bio/Technology 9:535). In general, direct gene transfer into protoplasts (using polyethyleneglycol treatment and/or electroporation) has seemed to have the best potential for success. Protoplasts for use in such direct gene transfer methods have most often been obtained from embryogenic cell suspension cultures (Lazzeri and Lorz (1988) Advances in Cell Culture, Vol.6, Academic press, p. 291; Ozias-Akins and Lorz (1984) Trends in Biotechnology 2:119). However, the success of such methods has been limited due to the fact that regeneration of phenotypically normal plants from protoplasts has been difficult to achieve for most genotypes.
Recently, success has been reported in the transformation of, and regeneration of phenotypically normal plants from, certain lines of rice (Shimamoto et al (1989) Nature 338:274; Datta et al (1990) Bio/Technology 8:736; and Hayashimoto et al (1990) Plant Physiol. 93:857) and corn (Gordon-Kamm et al (1990) Bio/Technology 2:603; Fromm et al (1990) Bio/Technology 8:833; Gould et al (1991) Plant Physiology 95:426; and PCT publications WO91/02071 and W089/12102). However, it is not clear from such reports that their processes of transformation and regeneration are applicable to monocots generally, particularly gramineous plants, quite particularly cereals.